In order to improve walking and running comfort, it has been proposed to provide shoe soles of elastomeric material in which gas-cushion cells are formed, these cells trapping a gas, generally air, at a superatmospheric pressure and forming compressible volumes with an elasticity brought about by the compressibility of the gas trapped therein.
Such soles can be composed of polyurethane or other elastomeric materials and include the elastic cushion which can extend over a large area or need merely be one of a number of cells or volumes of pressurized gas trapped in the sole.
The cushion can be formed upon an assembly of the sole from a number of parts.
While the cushioning effect has contributed to the comfort of such soles, the latter have not been fully satisfactory. With wear of the material, the sole in the region of the cushion or sole tends to bulge outwardly if the pressure of the cushion is retained and indeed, such bulging has been a problem. If, of course, the material in which the cushion or sole is formed is made of increased density or stiffness to prevent the bulging, the overall resiliency and cushioning effect is reduced. In practice, therefore, it has not been possible heretofore to reduce the outwardly bulging effect which has rendered earlier shoe soles of the trapped pressurized gas-cushion type ineffective.